Move release-environments child pipeline to be sourced from master

Part of https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/19905

In order to make development of release-environments easier by reducing
backports and skew of code across stable branches, we want to make it
so regardless of GitLab version being deployed to a release-environment,
we are always using the child-pipeline setup from master.

This means we can can make changes to the pipeline setup on `master`,
and it will take effect across all stable branches.

.gitlab/ci/release-environments.gitlab-ci.yml

@@ -20,4 +20,7 @@ start-release-environments-pipeline:
     PARENT_PIPELINE_ID: $CI_PIPELINE_ID
   trigger:
     strategy: depend
-    include: .gitlab/ci/release-environments/main.gitlab-ci.yml
+    include:
+      - project: 'gitlab-org/gitlab'
+        ref: 'master'
+        file: '.gitlab/ci/release-environments/main.gitlab-ci.yml'